Hotwire Measurements Downstream of a Delta Winglet Pair at Two Angles of Attack

Hotwire measurements were obtained downstream of a delta winglet pair placed on an unheated flat surface. Time-averaged mean velocity, RMS, fast Fourier transform and instantaneous velocity statistics are examined to gain insight into the effect that a delta winglet pair has on manipulating an otherwise steady baseline flow. Typically, results presented in the literature are in time-averaged form and this implies that the majority of information that relates to enhanced heat transfer, i.e. unsteady flow phenomena, is lost. It is for this reason that the current investigation examines the flow downstream of the vortex generator (VG) with hotwire anemometry so as to achieve good temporal and spatial measurement resolution. The mean velocity and RMS profiles presented at two different Angles of Attack (AoA) provide valuable information on the extent to which the VG manipulates the flow. In the centreline the boundary layer is significantly thinned in comparison to other spanwise locations indicating the presence of the downwash region. The shape of the mean velocity and RMS profiles also indicate the extent to which the vortex structures grow in the spanwise direction with downstream distance from the VG. The peak RMS values are shown to increase with downstream distance in some spanwise planes and decrease with downstream distance for other spanwise planes thereby illustrating complex fluid flow interactions. Examining the instantaneous flow features reveals the true nature of the unsteadiness and also elucidates some of the more complex flow phenomena, such as positive spikes found in the near-wall region, that may lead to enhanced heat transfer. It is also observed from the instantaneous velocity traces that large negative spikes are observed in the freestream region close to the boundary layer edge. These structures help to explain the interaction between the near wall and freestream flow field thereby resulting in significantly enhanced mixing.